Power transistors having high power handling capabilities are commonly employed as drive devices to supply relatively high levels of current to inductive loads. Examples of such inductive loads commonly encountered in the automotive industry include ignition coils and fuel injectors. In a so-called high-side driver arrangement, a power transistor is typically connected in series with the inductive load and upon the application of an excitation signal to the power transistor, a load current begins to flow through the inductive load and power transistor in a rapidly increasing manner. Upon removal of the excitation signal, a relatively high voltage is initially present at the connection between the power transistor and inductive load as the typically large load current flowing through the load begins to decay in accordance with the inductive properties of the load. Such a condition is often referred to as inductive flyback.
Unless otherwise provided for by a transient suppressor device, sometimes referred to as a snubber circuit, the high voltage present at the power transistor/inductive load interface may exceed the breakdown voltages of the various power transistor junctions. The combination of junction breakdown and resultant high load current flowing through the power transistor may result in a condition known as "latch-up", wherein a current path is established from the load to the substrate of the power transistor (ground potential). The resulting uncontrolled conduction of load current into the power transistor substrate may ultimately result in destruction of the power transistor.
Various techniques are known to restrain or suppress the detrimental action of the inductive flyback condition and some of these techniques may be described with reference to FIG. 1 herein. These techniques may employ a conventional power transistor driver 10, arranged as a high-side driver, in cooperation with flyback detection circuitry 12. The power driver 10 operates in a typical manner to supply load current from V.sub.SUPPLY to the inductive load L1 in response to an excitation signal, and after the excitation signal is removed from the power driver 10, the flyback detection circuit 12 operates in a typical manner to detect the inductive flyback condition and provide the necessary gating signals to render the power driver 10 conductive so as to return to its controlled "on" state. However, the cooperative action between the power driver 10 and flyback detection circuit 12 causes a relatively high level of power to be dissipated by the power driver 10 which may be expressed by the following relationship (1): EQU Power=ILOAD (V.sub.SUPPLY -(-VLOAD))
The power dissipated can be quite high since it is a function of the supply voltage V.sub.SUPPLY, as well as the load current ILOAD. Furthermore, the power dissipated could also be subjected to supply transients since the power driver 10 is connected directly to VSUPPLY. The power dissipated for transient suppression purposes can be lowered if a transient suppressor device 14 is used that would clamp or fix the operation of the additional device 14 at a definite level, such as a ground potential, and such lowered power dissipation may be expressed by the following relationship (2): EQU POWER=ILOAD (0-(-VLOAD))
The additional clamping device 14 should be comparable in size (current carrying capability) to the power driver 10 in order to handle the load current to which it is subjected. Such an additional device 14 may be in a form of a power diode, an auxiliary power transistor, or a self-triggered silicon controlled rectifier (SCR). The power diode option could provide the clamping function, but it would require a diode that has a relatively large silicon area so as to have the necessary high current carrying capability. The auxiliary power transistor option would improve the ability to provide for the needed silicon area, but the improvement in area would be lost by the need for additional drive circuitry for the auxiliary power transistor that operates in a manner similar to the flyback detection circuitry 12. The self-triggered SCR option would have the desired current carrying capabilities and not need additional drive circuitry, but it is difficult to implement a SCR with the type of fabrication processes used to provide analog, digital, and power circuits on a single integrated circuit.
All three (power diode, auxiliary power transistor, and SCR) of these configurations have an additional drawback possibility, that being the creation of a known parasitic diode in which the parasitic diode becomes forward biased under reverse V.sub.SUPPLY conditions and thereby injects supply current into the power driver 10. Furthermore, the power diode and auxiliary power transistor configurations may continuously allow current to be fed from the circuit ground back into the inductive load if a two-volt (2V) potential differential exists between the circuit ground of the integrated circuit and the load ground, as is common practice in applications involving inductive loads. For example, in many automotive applications, as much as a 2-volt potential difference may exist between circuit ground of any given "system" and chassis ground.
What is therefore needed is a combination flyback detection and transient suppressor device that provides an area efficient and low power means of snubbing inductive flyback without the risk of latch-up and without drawing current from the substrate of such a device. Ideally, such a device should be formed of a single integrated circuit and be capable of consistent operation when subjected to at least a 2V potential differential between circuit ground and load ground.